pages/*: add backticks around I/O streams (#10436)

* as suggested in #10428
* standard input  => `stdin`
* standard output => `stdout`
* standard error  => `stderr`

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/cmark.md

@@ -7,7 +7,7 @@
 
 `cmark --to html {{filename.md}}`
 
-- Convert data from standard input to LaTeX:
+- Convert data from `stdin` to LaTeX:
 
 `cmark --to latex`